Apolipoprotein A-I has been shown to be polymorphic in plasma and lymph and the major isoforms are designated apoA-I-3 and apoA-I-4. An additional isoform, apoA-I-1, was shown to increase in lymph following fat feeding. ApoA-I-1 was isolated from lymph and the amino-terminal sequence determined which revealed that apoA-I-1 was proapoA-I. ProapoA-I contains a hexapeptide (arg, his, phe, trp, gln, gln) attached to the amino-terminal end of apoA-I. ProapoA-I was also isolated and characterized from patients with Tangier disease. Amino-terminal sequence analysis of propoA-I-Tangier revealed that the propeptide was identical to normal proapoA-I. These data indicate that the rapid catabolism of apoA-I-Tangier is due to a structural defect in mature apoA-I-Tangier. The complete amino acid sequence of apoC-II from normal subjects has been completed. The apolipoprotein in 79 aminos in length and has several amino acid differences from the previously reported sequence of apoC-II from types IV and V subjects. The presence of B-48 and B-100 were determined in patients with a variety of dyslipoproteinemias. The combined results from this study indicated that B-48 is a metabolic marker for lipoprotein particles of intestinal origin and the elevation of plasma B-48 is observed in those dyslipoproteinemias characterized by defects in the metabolism of intestinal triglyceride-rich lipoproteins.